c. 1200, "hook-shaped instrument or weapon; tool or utensil consisting of or having as an essential component a hook or curved piece of metal," from Old Norse krokr "hook, corner," cognate with Old High German kracho "hooked tool," of obscure origin but perhaps related to the widespread group of Germanic kr- words meaning "bent, hooked." If there was an Old English *croc it has not been found.
From late 14c. as "a bend or curved part;" late 15c. as "any bend, turn, or curve." From mid-15c. as "a shepherd's staff with a curved top." Meaning "swindler" is American English, 1879, from crooked in figurative sense of "dishonest, crooked in conduct" (1708). Crook "dishonest trick" was in Middle English, especially in reference to the wiles of the Devil.
late 12c., "bend, cause to assume an angular or curved form," from crook (n.) or from an unrecorded Old English *crōcian. Intransitive sense of "to have a crooked shape, to bend or be bent" is from c. 1300. Crookback "hunchback" is from late 15c.